Even the most casual observer of the heavens cannot have failed to notice that in certain instances the stars are grouped so closely together as to form well-marked clusters. The most familiar example is the well-known group of the Pleiades, in the constellation Taurus, while quite close is the more scattered group of the Hyades. Another somewhat coarsely scattered group is that known as Coma Berenices, the Hair of Berenice, which lies beneath the handle of the Plough; and a fainter group is the cluster Præsepe, which lies in the inconspicuous constellation Cancer, between Gemini and Leo, appearing to the naked eye like a fairly bright, hazy patch, which the smallest telescope resolves into a cloud of faint stars. [Illustration: PLATE XXVIII. 1. 2. Irregular Star Clusters. Photographed by E. E. Barnard. 1. Messier 35 in Gemini. 2. Double Cluster in Perseus. ] The Pleiades form undoubtedly the most remarkable naked-eye group in the heavens. The six stars which are visible to average eyesight are Alcyone, 3rd magnitude; Maia, Electra, and Atlas, of the 4th; Merope, 4-1/3; and Taygeta, 4-1/2. While Celæno, 5-1/3; Pleione, 5-1/2; and Asterope, 6, hang on the verge of visibility. With an opera-glass about thirty more may be counted, while photographs show between 2,000 and 3,000. It is probable that the fainter stars have no real connection with the cluster itself, which is merely seen upon a background of more distant star-dust. Modern photographs have shown that this cluster is involved in a great nebula, which stretches in curious wisps and straight lines from star to star, and surrounds the whole group. The Pleiades make a brilliant object for a small telescope with a low magnifying power, but are too scattered for an instrument of any size to be effective upon them. The finest of all irregular star-clusters is that known as the Sword-handle of Perseus. Midway between Perseus and the W of Cassiopeia, and directly in the line of the Galaxy, the eye discerns a small, hazy patch of light, of which even a 2 or 3 inch glass will make a beautiful object, while with a large aperture its splendour is extraordinary. It consists of two groups of stars which are both in the same field with a small instrument and low powers. Towards the edge of the field the stars are comparatively sparsely scattered; but towards the two centres of condensation the thickness of grouping steadily increases. Altogether there is no more impressive stellar object than this magnificent double cluster (Plate XXVIII., 2). Another very fine example of the irregular type of grouping is seen in M. 35, situated in the constellation Gemini, and forming an obtuse-angled triangle with the stars Mu and Eta Geminorum (Plate XXVIII., 1). There are many other similar groups fairly well within the reach of comparatively small instruments, and some of these are mentioned in the list of objects (Appendix II.).